The present invention relates to a circuit configuration for an integrated semiconductor memory having a memory cell array with memory cells which are combined to form units of column and row lines, in which one of the column lines is accessed.
Integrated semiconductor memories have memory cells which are often configured in a memory cell array, for example in a matrix-type memory cell array. In this case, the memory cells are usually combined to form addressable units of column lines and row lines. These may be, for example, bit lines and word lines, respectively, at the crossover points of which the memory cells are configured.
In the event of a read or write access to one of the memory cells, the relevant row line and column line are generally selected via a decoder. After the selection of the relevant row lines, data signals of the memory cells along the row line are present on the corresponding column lines. These signals are amplified in an adjacent so-called sense amplifier strip of the memory cell array. After selection of the relevant column line, the data signal of the addressed memory cell is released.
In order to achieve the best possible access times when writing to and reading from an integrated memory, it is necessary that the signal portions for selection of the row lines and of the column lines be dimensioned to be as short as possible in the interests of a short signal propagation time. The selection of a row line or a word line generally requires more time than the selection of a column line or a bit line. With regard to access control, however, the row line must have already been activated prior to the access of the column line in order, as much as possible, to avoid signal disturbances during the writing or reading operation. The resulting requirement is that the memory cell must not be accessed faster from the column side than from the row side.
If the memory cell is accessed at a point in time that is determined by external access commands, it has been customary hitherto for starting the column decoding at a point in time that is temporarily delayed relative to the point in time at which the row decoding is started in accordance with the above requirements. Adherence to this temporal delay is guaranteed externally by a memory controller in customary standard specifications. In fast semiconductor memories (RLDRAM, FCRAM, etc.), the specification is altered by external commands to the effect that the row and column addresses are transferred simultaneously by the controller. In this case, the temporal control of column and row access must be supervised internally, for example by means of a delay circuit which models the difference between the column access time and the row access time.
A delay circuit of this type has, for example, an inverter chain or generally delay stages constructed from logic gates. However, modeling propagation times in such a way generally reduces the robustness of the integrated circuit. Delay elements of this type are comparatively sensitive to process, temperature, and voltage fluctuations. This may result in the delay circuit having delay times that are variable and/or are not precisely adjustable. As a result, the functionality of the semiconductor memory may be impaired in the event of a memory access.
It is accordingly an object of the invention to provide a circuit configuration for a semiconductor memory which overcomes the above-mentioned disadvantageous of the prior art semiconductor memories of this general type, which can be applied to the above-described integrated semiconductor memory as well as to a number of types of semiconductor memories, and in which the functionality of the circuit is ensured, as far as possible, independently from external influences.
With the foregoing and other objects in view there is provided, in accordance with the invention, an integrated semiconductor memory circuit configuration that includes memory cells which are configured in a memory cell array and which are combined to form addressable units of column lines and row lines. The circuit configuration furthermore has a decoder for selecting one of the column lines, which is connected to a column select line for transmission of a column select signal. The decoder furthermore has a terminal for an input signal for activating the column select signal. The circuit configuration additionally has a terminal for a row activation signal which serves for activating a row access signal sequence, for example, with the aid of a corresponding word line decoder. The terminal for the input signal of the column decoder is connected to a terminal for at least one signal from the row access signal sequence which indicates by its state that the row access is concluded.
Therefore, the column select signal is not activated and hence the relevant column line is selected before the access to the associated row line is concluded. This means that, in the memory access process, the successive process steps of row decoding and column decoding are controlled by successive select signals. The memory access process thus acquires a self-adjusting method of operation.
In the case of a memory access of this type, the influence of the technology on the signal propagation time can thus be taken into account in a self-controlling manner. If the memory access control is clocked, for example, the successive memory access process steps described are independent of the clock frequency. The modeling of the signal propagation times by delay elements is obviated, thereby avoiding the susceptibility to process, temperature or voltage fluctuations.
In accordance with an added feature of the invention, the circuit configuration has a sense amplifier, which is assigned to one of the memory cells of a selected row line, with a terminal for an activation signal. The activation signal is part of the row access signal sequence. In the course of the row access, the relevant sense amplifier is activated and hence the row access is ended. Consequently, the activation signal of the sense amplifier is advantageously suited to activating the column select signal.
In accordance with an additional feature of the invention, it is possible for the terminal for the input signal of the decoder for activating the column select signal to be connected to a signal derived from the activation signal of the sense amplifier.
In accordance with another feature of the invention, the circuit configuration has a storage element with a set input, a reset input and an output. By way of example, a signal which releases the column access is present at the set input. Moreover, the circuit configuration has a logic gate which is connected to the output of the storage element and to the terminal for the signal from the row access signal sequence. The access signals for the column access and the row access are thus logically combined with one another. An output of the logic gate is connected to the terminal for the input signal of the decoder.
In accordance with a concomitant feature of the invention, the invention can advantageously be used in fast semiconductor memories such as, for example, RLDRAM or FCRAM, since the fastest possible propagation times in conjunction with high functional reliability are made possible by the self-adjustment.
Although the invention is illustrated and described herein as embodied in a circuit configuration for an integrated semiconductor memory with column access, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.